Abstract
Interaction of Ca and acetylcholine (AcCh) ions with purified acetylcholine receptor (AcChR) from Torpedo californica and Electrophorus electricus has been investigated in view of these ions' role proposed in bioelectricity. Spectrophotometric Ca titration using murexide as an indicator and an ultrafiltration method with 45Ca show that AcChR proteins have a high binding capacity for Ca ions. Per macromolecule of 260,000 daltons, up to 60 Ca ions can be bound with at least three Ca dissociation constants. A linear inhibition of AcCh binding to AcChR by Ca was observed in the 0.1-1 mM Ca range, indicating competition of AcCh and Ca for AcChR. The addition of AcCh to a Ca-AcChR solution at 1.2 mM Ca causes release of four to six bound Ca ions from AcChR when a maximum of two AcCh ions are bound per 260,000 dalton macromolecule. The subsequent addition of alpha-bungarotoxin causes reuptake of up to six Ca ions by AcChR. These results suggest that the neural activator AcCh and the inhibitor alpha-bungarotoxin induce opposing shifts between different conformational states of isolated AcChR.
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